1. Field of the Invention
This invention pertains to methods for producing polyacrylonitrile derivatives by a controlled heterogeneous reaction.
2. Background of the Related Art
Copolymers containing predominantly acrylonitrile units have been synthesized for a number of purposes. For example, to improve the reception of dyes of acrylic fibers, to improve polyacrylonitrile (PAN) membrane characteristics, and to produce water-absorptive or water-soluble polymers.
Certain products of PAN reactions are known to have sequential block compositions, so that the residual nitrile groups are organized in continuous sequences, as are the groups resulting from the respective reaction of the nitrile groups. The average length of such sequences or blocks is larger than the length of sequences corresponding to random group distribution at the same reaction conversion. This is due to the so-called "zipper mechanism," in which a newly formed group activates neighboring nitrile groups. While the reaction starts on randomly selected nitrile groups, once new groups are formed, any additional new groups are more likely to be formed in the vicinity of previously formed groups so that the sequence of newly formed groups grows gradually, resembling a "zipper opening". The result of such a reaction is a block copolymer with continuous sequences or blocks of the original nitrile groups and newly formed groups (e.g. amides). Because each polymer contains a multitude of blocks of each kind, the product is called a "multiblock copolymer" (MBC). The process for forming a MBC and the general characteristics of MBC's have been described in articles by various authors, for example, N. A. Plate, et al., in the Journal of Polymer Science, Vol. 12, 2165-2185 (1974).
Prior to this invention, the only known method for producing MBC's of PAN was by hydrolysis of homogeneous solutions of PAN, in the presence of either an acid or a base catalyst, to yield PAN-acrylamide or PANacrylic acid MBC's, respectively. A "homogeneous" solution of PAN is a solution in which the PAN polymer chains are fully solvated by a "good" PAN solvent, i.e. a solvent whose Flory-Huggins .xi. factor is less than 0.5, so that all chains are equally accessible to the solvent. Other solution reactions of PAN, i.e. Radyszewski or Ritter reactions, as described by V. Janout, Ph.D. Thesis, Institute of Macromolecular Chemistry, Prague, C.S.S.R. (1978), only yield random copolymers.
Homogeneous hydrolysis reactions of PAN were first described by Polson in U.S. Pat. No. 2,579,451 in 1951, again in U.S. Pat. Nos. 2,721,133, 3,368,015 and 3,926,930 by Downing, et al, Irion, et al and Olijuka, et al, respectively. However, none of these inventors realized that the reaction product was a MBC.
In most cases, highly concentrated acid is used as both the PAN solvent and catalyst. Stoy, et al in U.S. Pat. No. 3,897,382 disclosed that improved control of hydrolysis of PAN in acids results where the hydrolysis reaction is first run at elevated temperatures to generate some randomly distributed amidic groups, and then finished at low temperatures to generate continuous sequences. Initiating amidic groups have also been incorporated into starting PAN by copolymerization, see also U.S. Pat. No. 3,948,870 issued to Stoy, et al.
Alkaline-catalyzed solution hydrolysis is described by Stoy in U.S. Pat. No. 4,017,121 where PAN is dissolved in a concentrated aqueous solution of NaSCN and then hydrolyzed in the presence of a basic catalyst, such as sodium hydroxide. This reaction leads predominantly to acrylonitrile-acrylic acid multiblock copolymers. By contrast, acid-catalyzed hydrolysis of PAN predominantly yields acrylonitrile-acrylamide copolymers.
In the presence of certain catalysts, PAN hydrolysis in solution causes primarily formed amidic groups to form glutaramide groups by condensation of neighboring amides, as described by Stoy in U.S. Pat. Nos. 4,331,783 and 4,369,289. As further described in detail by Stoy in U.S. Pat. Nos. 4,370,451 and 4,337,327, the resulting copolymers with glutaramide sequences can then be converted by variety of known reactions into new block copolymers which cannot be prepared by direct hydrolysis.
Hydrolysis of PAN dissolved in acids is also described in non-patent literature, such as, C. W. Saltonstall, et al., R&D Progress Report #220, Department of Interior (November, 1966); Soler J. Baldrian, and J. Angew, Macromol Chem. 49, 49 (1976); Janacek, et al., J. Polymer Sci., Symposium No. 53, 2SS (1975); and Lovy, J., et al., Coll. Czech, Chem. Commun., 506 Vol. 49 (1984). Eventually, investigators gradually recognized that such PAN hydrolysis reactions yield multiblock acrylonitrile copolymers.
An alternative method of homogeneous acidic hydrolysis of PAN using covalently cross-linked PAN gels swollen with acidified ZnCl.sub.2 or HNO.sub.3 is described by Stoy and Stoy, et al., respectively, in U.S. Pat. Nos. 4,123,406 and 4,228,056. From the viewpoint of hydrolysis kinetics and mechanism, there is no substantial difference between an acid solution of PAN and crosslinked PAN swollen in the same acid. All nitrile groups are similarly accessible and solvated, regardless of the presence of infrequent chemical links between polymer chains. This is why the term "homogeneous hydrolysis" covers both solutions of PAN and cross-linked PAN swollen in reactive solvents of PAN.
Homogeneous acidic hydrolysis of PAN can also be carried out in a mixture of both dissolved (i.e. reactive) and undissolved (i.e. inert) PAN. This reaction was described by Stoy in U.S. Pat. Nos. 3,864,323 and 4,379,874 as "heterogeneous". However, such hydrolysis is "heterogeneous" only in the sense that various fractions of PAN start hydrolysis at different times so that the product is heterogeneous with a broad or bimodal distribution of conversions per chain. From the viewpoint of the hydrolytic reaction, it is again carried out in solutions, i.e., in a state of full solvation of reacting nitrile groups.
A heterogeneous process in the true sense of the word is where solid PAN is dispersed in the reaction medium, and the reaction is carried out in the absence of PAN solvents. In most cases, the reaction is the basecatalyzed hydrolysis of solid PAN. The product of such hydrolysis can either be water-soluble acrylic-acrylamide copolymers (see e.g., U.S. Pat. Nos. 2,812,317, 2,861,059 and 3,200,102 issued to Barret, Mow and Kleine, respectively), or cross-linked water swellable particles or fibers (see, e.g., U.S. Pat. Nos. 4,272,422 and 4,366,206 issued to Tanaka). In the case of the watersoluble products, they dissolve as they form to expose fresh unreacted PAN surfaces.
Alkaline hydrolysis of nitrile groups can also be carried out on the surface of densely cross-linked copolymers as described by Hradil, et al., in U.S. Pat. No. 3,964,973.
Acid-catalyzed heterogeneous reactions of PAN are much less common and are usually carried out only on the PAN surface, for instance, by reaction with sulphuric acid to introduce sulfo-and sulfate groups while simultaneously stabilizing the surface layer by crosslinking, see U.S. Pat. No. 3,895,169 issued to Wichterle. Another surface reaction on solid dry PAN is the sulfonation by gaseous S03 as described by Takezo, et al., U.S. Pat. No. 4,265,959, or the plasma-activated surface oxidation used to produce semi-permeable membranes, as described by Takezo, et al. in U.S. Pat. No. 4,107,049. However, none of these reactions yields multiblock copolymers, except perhaps in an extremely thin and inseparable surface layer. Attempts at heterogeneous acidic hydrolysis which could provide multiblock hydrolyzates have been unsuccessful, see C. W. Saltstall, et al supra.
Accordingly, prior to this invention, only acidcatalyzed hydrolysis of PAN carried out in the presence of a reactive PAN solvent was known to yield PAN multiblock copolymers, whether cross-linked or non cross-linked and whether in pure form or as composites with PAN.
Other multiblock derivatives of PAN have been prepared indirectly, i.e., by conversion of primary acrylonitrile-acrylamide multiblock copolymers formed by homogeneous acid-catalyzed hydrolysis. Such copolymers are recognizable by the distribution of functional groups reflecting the structure of the parent glutaramide copolymer. Examples of processes for indirect preparation of PAN derived MBC's, include: methods using amide cyclization to form glutarimide derivatives which are subsequently cleaved, as described by Stoy in U.S. Pat. Nos. 4,331,783 and 4,369,289; alkaline treatment of products of acid-catalyzed hydrolysis, as described by Stoy in U.S. Pat. Nos. 4,337,327 and 4,370,451; Bouvault reaction between amide groups and nitrous acid in acrylonitrile-acrylamide multiblock copolymers, as described in U.S. Pat. Nos. 4,123,406 and 4,480,642 issued to Stoy and Stoy, et al., respectively; and, surface sulfonation or sulphatation of amides while simultaneously cross-linking the product by using, for example, glycerol, as described by Wichterle, et al., in U.S. Pat. No. 4,183,884.
The structure of PAN in the solid state is very different from most polymers. One major difference is that solid-state PAN is fully crystalline: absent is an amorphous phase, which always coexists with the crystalline phase in other polymers The second major difference is that in stretched or oriented PAN the crystalline phase is organized perpendicular to the stress vector, while very little or no order is detectable in the direction of stress. The third major difference is that PAN crystallizes to the same extent and with the same morphology regardless of tacticity See, Bohn, C. R., et al., J. Polymer Sci. 55, 531 (1961) for a detailed description of the structural features of PAN in the solid state.
PAN is an excellent barrier to the diffusion of low molecular weight compounds, due to its very high crystallinity and the absence of an amorphous phase. Also, all (or nearly all) of the strongly dipolar nitrile groups of PAN are incorporated in the crystalline matrix. Accordingly, solid PAN is very reluctant to engage in a chemical reaction unless it is in the presence of a PAN solvent. This high crystallinity and the very low permeability of PAN to reagents explains why PAN reacts mostly in the presence of solvents which dissolve its crystalline phase. It is also the reason why solid state PAN only reacts with bases to form water soluable or cross-linked products. Namely, such a reaction has to proceed in a layer-by-layer fashion allowing the reacted portions to be dissolved, unless the polymer is covalently cross-linked.
Prior to this invention, only two reactive states of PAN were contemplated, namely, the solid state where all reactive groups are bound within a dense crystalline matrix, and the dissolved or plasticized gel state where the crystalline phase has been broken down by the solvation of nitrile groups in a solvent of PAN.
A third state has been known, but it has never been closely studied or contemplated as a special reactive state of PAN. This third state is hereinafter referred to as the "aquagel" state or "AQG". The AQG is a metastable structure in which PAN contains water or another liquid. However, AQG is not PAN swollen to equilibrium, rather, decrease in the liquid content of PAN AQG is irreversible. There are substantial differences in the properties of the AQG state of PAN from the solid state crystalline, or the dissolved or plasticized gel state of PAN.
The present invention is based on the special properties of the PAN aquagel state. However, since the PAN aquagel state has not been satisfactorily studied or described prior to this invention, we have included the following description of the properties of the PAN aquagel state and cited the references we consider pertinent.
Some investigators have observed that PAN solutions in water-miscible solvents can be coagulated by contact with water. The coagulation is, in fact, precipitation of PAN in the solid form by exchanging a "good" PAN solvent, i.e., a solvent having a Flory-Huggins factor&lt;0.5 and capable of fully solvating the PAN chains for a "poor" PAN solvent, i.e., a solvent having a Flory-Huggins factor&gt;0.5 and incapable of fully solvating the PAN chains (e.g., water or mixtures of the good solvents with water). These good PAN solvents include several inorganic compounds in concentrated aqueous solution, including calcium thiocyanate, sodium thiocyanate, zinc chloride, lithium bromide, magnesium perchlorate, phosphoric acid and nitric acid. Also, several organic compounds in substantially anhydrous solutions are good PAN solvents, including dimethylsulfoxide, dimethylformamide, gamma-butyrolactone, tetramethylenesulfone and dimethylacetamide. Alternatively, other water miscible fluids, such as low aliphilic alcohols containing one to four carbon atoms per molecule, and the like, which are incapable of dissolving PAN can be used to coagulate PAN. By a "fluid" we mean either a liquid or a gas. The coagulation process is used in the wet spinning of acrylic fibers and in the casting PAN membranes, where coagulation is often preceded by partial evaporation of the solvent to create a surface "skin".
Coagulation is a complicated physical process involving diffusion of water or another coagulant into the PAN solution, and diffusion of the PAN solvent out of the solution. As the solvent's thermodynamic ability to hold PAN in solution deteriorates, due to the increasing water content, PAN precipitates into a new solid phase. The new PAN solid phase hinders the diffusion of all permeants. Because water permeates into the precipitating polymer faster than the solvent diffuses out, a number of small osmotic cells are formed having precipitated PAN polymer walls. The result is a heterogeneous, porous or microporous composition formed by the precipitated PAN polymer matrix with voids or pores filled with water. Because the differences between the diffusion rates of water and the solvent increases with increasing length of the diffusion path, a skin is formed on the surface of the coagulated article which is usually less heterogeneous and porous than the interior "bulk" of the coagulated article. This skin formation is used in the production of membranes with asymmetric structures in which the porous bulk provides high flow rates while the denser skin provides for separation of permeants by their diffusion rates, or by the size of their molecules in solution which is related to their diffusion coefficients.
Processes for producing semi-permeable PAN membranes are described in U.S. Pat. Nos. 4,272,378, 4,147,745, 4,265,959, 4,107,049, 3,975,478, and 4,268,662. However, probably because the investigators looking at membranes were interested in creating porous structures, the coagulated PAN polymers described in these membrane related patents were thought of as any other porous articles, in which the pore walls were composed of PAN. These investigators never contemplated or attempted to explain the structure and character of the solid-phase PAN product in these structures. The differences in permeabilities and flow rates were explained in terms of water-filled pores of varying size distribution. There is nothing in the aforesaid patents which either teaches or suggests that the structure and properties of the coagulated PAN would differ from the structure and properties of PAN in a homogeneous article, such as an acrylic fiber or a dry membrane cast without coagulation, for instance by evaporation of the PAN solvent.
A PAN article prepared by coagulation with subsequent drying has the same polymer properties including, Tg, crystallographic structure, thermodynamic properties, etc., as the article made by solvent evaporation, e.g., solvent cast membrane or dry-spun fiber. Both articles are non-melting, non-swellable in water, and orientable by stretching at high temperatures. In each case PAN acts as an excellent barrier to the diffusion of low-molecular weight solutes, because it is only soluble in a few highly polar solvents, and is laterally organized in the oriented state.
When the coagulation process has been used in fiber-spinning to prepare fibers without macroscopic pores, the coagulated PAN product has been referred to as an "Aquagel". The coagulated fiber cannot have microscopic pores, since it must be stretched to orient the polymer chains, to strengthen the fiber. Aquagels are described as made from solutions of polymers or copolymers of acrylonitrile containing at least 85% acrylonitrile units in aqueous concentrated solutions of certain salts such as zinc chloride or sodium thiocyanate. If such solutions are spun into ice-cold water at temperatures not exceeding about 2.degree. C., the obtained "Aquagel" filament can be easily drawn in hot water, forming a good, strong acrylic fiber. See, for example, U.S Pat. Nos. 2,558,730 to 2,558,735 issued to Cresswell, et al. Under these conditions a gelled fiber without macroscopic pores is formed. This fiber contains a certain amount of water, and can be easily stretched in hot water prior to drying. Stanton, et al. in U.S. Pat. No. 2,790,700 describe a process in which the fiber's heterogenicity is suppressed by coagulation in a dilute PAN solvent. In Stanton, et al's methods, the solvent gradient is lowered slowing down the rate of coagulation so that osmotic pore formation is suppressed.
Cummings, Jr. in U.S. Pat. No. 2,948,581 describes a similar fiber coagulation process using thiocyanate instead of zinc chloride as the PAN solvent. It is believed that the gelled state and easy stretching reported by Cummings, Jr. is due to residual thiocyanate solvent in the gelled fiber which plasticizes the polymer to a limited extent. After stretching, the fiber is washed removing the residual thiocyanate solvent. Likewise, U.S. Pat. No. 3,689,621 issued to Fuji, et al. describes the process of coagulation of PAN to an aquagel from a zinc chloride solution. The process is modified by combined cold and hot stretching in the aquagel and dried states.
Fukisaki, et al. in U.S. Pat. Nos. 3,073,669 and 3,080,209 describe the process of dissolving PAN in concentrated nitric acid and the coagulation of PAN fibers in dilute nitric acid solution. The process includes the steps of wet fiber spinning and fiber stretching prior to drying. Also, U.S. Pat. No. 3,080,209 includes the addition of univalent cations to the coagulation bath to improve the dyeability of PAN fibers when using basic dyestuffs. The diffusion of water-soluble compounds such as dyestuffs into the PAN is much higher before drying than after the PAN aquagel has been dried. In addition, the high permeability cannot be restored once PAN has been dried. At the time the Fujisaki, et al patents were published, the accepted explanation for the properties of PAN AQG was that the aquagel state is a metastable state in which PAN crystallization gradually takes place, as the water content gradually decreases. In addition, it was assumed that the aquagel state is simply a state in which there are very small water filled voids in the PAN structure. It is believed that these voids irreversibly collapse by drying or by evaporation over an extended period of time as residual solvent is removed.
In addition, coagulation of PAN solution can be achieved by non-aqueous fluids which are miscible with water, such as lower aliphatic alcohols or glycols, which are subsequently replaced with water, see, C. W. Saltonstall, et al.: Research and Development of New Polymer System for Reverse Osmosis Membranes: R&D Progress Report #167, Department of Interior (February, 1966) and Ibid. R&D Progress Report #220 (November, 1966).
The aquagel state of PAN is also referred to as a gelled state, gel or hydrogel. However, by definition an aquagel is not a hydrogel, since a hydrogel swells reversibly in water, while an aquagel cannot regain its water content once it is dried. The structure and properties of the aquagel state have never been thoroughly studied prior to this invention although, as demonstrated above, it has practical utility. To date, aquagel is considered microporous or ultra microporous polyacrylonitrile. The term "aquagel", prior to this invention, referred without exception to clear translucent PAN coagulates. While PAN coagulates with permanently visible pores were merely deemed to be porous PAN.
The aquagel state of PAN can be discerned from plain porous dried PAN by immersing a sample into an aqueous solution of a dyestuff such as methylene blue. Aquagel is rapidly stained and the hue remains intact when the sample is dried. The coloration cannot be removed by washing in cold water. By contrast, the plain porous dried PAN does not become stained, and any dyestuff, caught perhaps at the surface, can easily be washed off in cold water.
The aforementioned references emphasized that very specific and narrow coagulation conditions are necessary to obtain PAN in the aquagel state. These references also teach that coagulation of PAN under conditions outside those very narrow limits results in porous PAN.
However, contrary to the accepted teachings in the art, discussed above, we have found that the "aquagel" state of PAN is not only formed in the narrow range of conditions previously believed necessary, but rather over a very broad range of conditions which exist during the replacement of any PAN solvent for water. Furthermore, the porosity observed under most coagulation conditions does not appear to be directly related to presence or absence of the aquagel state.
An important indication that aquagel is not merely simple microporous PAN, as was previously thought, is that the plasticizing effect is observed with the nonpolar, water-immiscible liquids such as hexane, toluene, silicone oil, poly (propylenoxide), mineral oil, chlorinated hydrocarbons, organic phtalates and phosphates, as well as with polar, water-miscible liquids, such as dimethylsulfoxide, dimethylformamide, gammabutyrolactone, tetramethylene, sulfone, dimethylacetamide, glycerol and 1,2-propylene glycol, and many others. This shows that aquagel is a peculiar state of the PAN polymer which is neither swollen, nor simply microporous with the pores filled with a liquid phase. If aquagel were merely simple microporous PAN and if aquagel were swollen, then the swelling should be dependent on thermodynamic parameters of the liquid, such as solubility parameters or cohesion energy. This is clearly not the case, rather, aquagels retain their volume irrespective of the character of the liquid. Also, if aquagels were porous structures, then there would be no plasticizing effect by the liquids, which are all poor solvents of PAN with a Flory-Huggins .xi. factor&gt;0.5. Finally, if aquagels were a porous structure with very small pores filled with liquids, they would have a large wetting angle making them thermodynamically unstable, which is clearly not the case.
Contrary to accepted teachings in the art, we have found that aquagels made with both water-miscible and water immiscible liquids are stable and stay plasticized over very long periods of time without any change in their properties. Aquagels retain their stability as long as the liquid is not removed, e.g., by evaporation. Accordingly, the plasticizing effects of water-immiscible liquids on polyacrylonitrile having the structure acquired in the "aquagel" state are very surprising and cannot be satisfactorily explained by the current theoretical concepts of polymer swelling, or of porous, glassy polymers.